"Gary has trained in the gym and came through fine," Mowbray told BBC Tees.
"He's been on a programme to strengthen the areas where he has had problems so if there are no after-effects he'll be in contention."
O'Neil has been plagued by injuries since his move to the Riverside from Portsmouth in August 2007, having suffered knee, groin and hamstring problems in recent seasons.
The former England under-21 international has made 12 appearances this season but is yet to open his scoring account.
Meanwhile, Mowbray has also dismissed claims that goalkeeper Jason Steele and defender David Wheater are set for a departure from Teesside, in the light of reported interest from Premier League sides Arsenal and Newcastle United respectively.
"I give short shrift to speculation in newspapers about footballers, as a manager I've had it for the last six or seven years," Mowbray added.
"In my first job at Hibernian every player was being sold in the first few weeks I got there, it never materialised.
"Journalism is what it is, people have to sell newspapers and have to fill column inches, people have a job to do and are under pressure from their editors so they sometimes smudge the issues or a rumour gets started to get a story in there.
"The only time those things become serious to me is when the phone rings and a manager is asking about a player's availability and that hasn't happened."
Source: BBC Sport
Source: BBC Sport